In the field of non-volatile memories, ReRAM (resistive RAM) is currently spotlighted. A ReRAM device is a kind of variable-resistance element, and generally includes a pair of electrodes and a recording film that can be selectively switched between the high resistance state and the low resistance state, according to a voltage applied between the pair of electrodes. ReRAM devices allow recording or rewriting data utilizing the selective switching of the resistance state of the recording film. The following patent documents 1 to 3 disclose such kind of ReRAM devices and variable-resistance elements.                Patent document 1: JP-A-2004-273615        Patent document 2: JP-A-2004-281913        Patent document 3: JP-A-2005-123361        
ReRAM devices are broadly classified into a bipolar type and a unipolar type, from the viewpoint of electrical characteristics. In a bipolar ReRAM device, the direction of voltage application between the pair of electrodes for switching the recording film from the high resistance state to the low resistance state is different from the direction of voltage application between the pair of electrodes for switching the recording film from the low resistance state to the high resistance state. Accordingly, in a bipolar ReRAM device, the voltage of different polarities is employed when changing or switching the two types of resistance states. On the other hand, in a unipolar ReRAM device, the direction of voltage application between the pair of electrodes for switching the recording film from the high resistance state to the low resistance state is the same as the direction of voltage application between the pair of electrodes for switching the recording film from the low resistance state to the high resistance state. Accordingly, in a unipolar ReRAM device, the voltage of the same polarity is employed when changing the two types of resistance states. Since unipolar ReRAM devices allow applying the voltage of the same polarity when switching the two different resistance states (in other words, there is no need to change the polarity of the applied voltage when switching the two different resistance states) a unipolar ReRAM device is more advantageous compared with a bipolar ReRAM device, in that a unipolar ReRAM device facilitates avoiding complication of the device circuit including a ReRAM device.
Unipolar ReRAM devices including a recording film constituted of NiO, or a recording film constituted of TiO2 have been reported. Regarding these ReRAM devices, however, although the fact that the unipolar-type operation is possible is known, the working principle has not been specified. Since the working principle is unclear, orientation for selection of materials for the parts of the ReRAM devices or for optimization of the design size cannot be established, which impedes the optimization in designing the ReRAM devices. Besides, presumably, employing different base materials for the recording film leads to a substantial change in working principle of the ReRAM device.
The present invention has been proposed under the above described situation, and it is therefore an object of the present invention to provide a variable-resistance element capable of performing a unipolar type operation under a predetermined working principle, and suitable for operating with a low voltage.